System and method for determining rotor position offset of an electric machine

ABSTRACT

A method according to an exemplary aspect of the present disclosure includes, among other things, detecting a position, including a polarity, of a rotor to detect incorrect rotor position offset of an electric machine without generating torque or motion within the electric machine.

TECHNICAL FIELD

This disclosure relates to electric vehicles, and more particularly, butnot exclusively, to a system for determining rotor position offset of anelectric machine of an electrical vehicle.

BACKGROUND

Hybrid electric vehicles (HEV's), plug-in hybrid electric vehicles(PHEV's), and battery electric vehicles (BEV' s) (hereinaftercollectively referred to as “electric vehicles”) differ fromconventional motor vehicles in that they employ one or more electricmachines in addition to an internal combustion engine to drive thevehicle. Electric vehicles may also be equipped with a battery thatstores electrical power for powering the electric machines. In someelectric vehicles, an electric machine may also be employed as agenerator that is powered by the internal combustion engine in order togenerate electrical power to charge the battery.

Electric machines may incorporate synchronous motors having a stator anda rotor with permanent magnets. It may become necessary to determine aposition of the rotor in order to meet electric motor controlrequirements of the electrical vehicle and to avoid inaccurate torqueproduction.

SUMMARY

A method according to an exemplary aspect of the present disclosureincludes, among other things, detecting a position, including apolarity, of a rotor to detect incorrect rotor position offset of anelectric machine without generating torque or motion within the electricmachine.

In a further non-limiting embodiment of the foregoing method, the stepof detecting includes applying a voltage and analyzing a currentresponse from the electric machine to determine the position, includingthe polarity, of the rotor.

In a further non-limiting embodiment of either of the foregoing methods,the step of detecting includes applying a first voltage of a firstmagnitude and a first frequency to the electric machine to produce afirst current response and processing a first current response from theelectric machine to determine an alignment of a direct axis of therotor. The step includes applying a second voltage of a second magnitudeand a second frequency to the electric machine and analyzing a secondcurrent response from the electric machine to determine the polarity ofthe direct axis of the rotor.

In a further non-limiting embodiment of any of the foregoing methods,the first magnitude is a different magnitude from the second magnitude.

In a further non-limiting embodiment of any of the foregoing methods,the first frequency is a different frequency from the second frequency.

In a further non-limiting embodiment of any of the foregoing methods,the step of processing includes processing a negative sequence currentresponse from the electric machine.

In a further non-limiting embodiment of any of the foregoing methods,the first voltage is a sinusoidal rotating voltage and the secondvoltage is a sinusoidal pulsing voltage along the direct axis.

In a further non-limiting embodiment of any of the foregoing methods,the method comprises one of the steps of determining that the polarityof the direct axis is correct if the average value of the first currentresponse is a positive value or determining that the polarity of thedirect axis is incorrect if the average value of the first currentresponse is a negative value.

In a further non-limiting embodiment of any of the foregoing methods,the method comprises the step of adjusting the position by 180° if theaverage value of the first current response is the negative value.

In a further non-limiting embodiment of any of the foregoing methods,the method comprises the step of comparing the position and the polarityof the direct axis of the rotor to information from a sensor configuredto monitor the electric machine to determine the incorrect rotorposition offset.

In a further non-limiting embodiment of any of the foregoing methods,the method comprises the step of taking a corrective action if the rotorposition offset is out of range.

In a further non-limiting embodiment of any of the foregoing methods,the step of detecting is performed in response to a predefined prompt.

In a further non-limiting embodiment of any of the foregoing methods,the predefined prompt is a key-on condition of an electric vehicle.

In a further non-limiting embodiment of any of the foregoing methods,the step of detecting includes using a voltage command tracking method.

In a further non-limiting embodiment of any of the foregoing methods,the step of detecting includes comparing back electromotive force (EMF)to position signal.

A method according to another exemplary aspect of the present disclosureincludes, among other things, detecting an incorrect rotor positionoffset of an electric machine using a decaying sinusoidal torque.

In a further non-limiting embodiment of the foregoing method, using thedecaying sinusoidal torque includes applying a first current of a firstmagnitude to a direct axis of a rotor of the electric machine, applyinga second current of a second magnitude to a quadrature axis of therotor, tapering the second current to a configurable amplitude at acalibratable taper rate, and filtering a position response of the rotorto identify a zero position.

In a further non-limiting embodiment of either of the foregoing methods,the step of tapering includes using one of a linear ramp and anexponential decay.

A rotor position offset detection system according to an exemplaryaspect of the present disclosure includes, among other things, anelectric machine having a rotor, a sensor that monitors a position ofthe rotor and a control unit in communication with the sensor. Aninverter is in communication with the control unit. The control is unitconfigured to compare information from the sensor with feedback from theinverter to detect an incorrect rotor position offset of the rotor.

In a further non-limiting embodiment of the foregoing system, thecontrol unit is configured to command a 3-phase voltage to the inverter.

The various features and advantages of this disclosure will becomeapparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a powertrain of an electric vehicle.

FIG. 2 illustrates part of an electric drive system of an electricvehicle.

FIG. 3 illustrates a mathematical model of an electric machine rotorrelative to a direct axis and a quadrature axis.

FIG. 4 illustrates a rotor position offset detection system that can beincorporated into an electric vehicle.

FIG. 5 illustrates a first embodiment of a method for determining rotorposition offset of an electric machine.

FIG. 6 illustrates a second embodiment of a method for detecting rotorposition offset of an electric machine.

FIG. 7 illustrates a third embodiment of a method for determining rotorposition offset of an electric machine.

FIG. 8 illustrates a fourth embodiment of a method for determining rotorposition offset of an electric machine.

DETAILED DESCRIPTION

This disclosure relates to a system and method for determining a rotorposition offset of an electric machine employed within an electricvehicle to meet electric motor control requirements and to avoidinaccurate torque production by the electric machine. The system andmethods disclosed herein provide an in-vehicle diagnostic method fordetecting incorrect rotor position offset without the need to undertakerelatively costly and time consuming maintenance operations.

FIG. 1 schematically illustrates a powertrain 10 for an electric vehicle12, such as a hybrid electric vehicle (HEV). Although depicted as a HEV,it should be understood that the concepts described herein are notlimited to HEV' s and could extend to other electric vehicles, includingbut not limited to, plug-in hybrid electric vehicles (PHEV's) andbattery electric vehicles (BEV's).

In one embodiment, the powertrain 10 is a powersplit powertrain systemthat employs a first drive system that includes a combination of anengine 14 and a generator 16 (i.e., a first electric machine) and asecond drive system that includes at least a motor 36 (i.e., a secondelectric machine), the generator 16 and a battery 50. For example, themotor 36, the generator 16 and the battery 50 may make up an electricdrive system 25 of the powertrain 10. The first and second drive systemsgenerate torque to drive one or more sets of vehicle drive wheels 30 ofthe electric vehicle 12, as discussed in greater detail below.

The engine 14, such as an internal combustion engine, and the generator16 may be connected through a power transfer unit 18. In onenon-limiting embodiment, the power transfer unit 18 is a planetary gearset. Of course, other types of power transfer units, including othergear sets and transmissions, may be used to connect the engine 14 to thegenerator 16. The power transfer unit 18 may include a ring gear 20, asun gear 22 and a carrier assembly 24. The generator 16 is driven by thepower transfer unit 18 when acting as a generator to convert kineticenergy to electrical energy. The generator 16 can alternatively functionas a motor to convert electrical energy into kinetic energy, therebyoutputting torque to a shaft 26 connected to the carrier assembly 24 ofthe power transfer unit 18. Because the generator 16 is operativelyconnected to the engine 14, the speed of the engine 14 can be controlledby the generator 16.

The ring gear 20 of the power transfer unit 18 may be connected to ashaft 28 that is connected to vehicle drive wheels 30 through a secondpower transfer unit 32. The second power transfer unit 32 may include agear set having a plurality of gears 34A, 34B, 34C, 34D, 34E, and 34F.Other power transfer units may also be suitable. The gears 34A-34Ftransfer torque from the engine 14 to a differential 38 to providetraction to the vehicle drive wheels 30. The differential 38 may includea plurality of gears that enable the transfer of torque to the vehicledrive wheels 30. The second power transfer unit 32 is mechanicallycoupled to an axle 40 through the differential 38 to distribute torqueto the vehicle drive wheels 30.

The motor 36 can also be employed to drive the vehicle drive wheels 30by outputting torque to a shaft 46 that is also connected to the secondpower transfer unit 32. In one embodiment, the motor 36 and thegenerator 16 are part of a regenerative braking system in which both themotor 36 and the generator 16 can be employed as motors to outputtorque. For example, the motor 36 and the generator 16 can each outputelectrical power to a high voltage bus 48 and the battery 50. Thebattery 50 may be a high voltage battery that is capable of outputtingelectrical power to operate the motor 36 and the generator 16. Othertypes of energy storage devices and/or output devices can also beincorporated for use with the electric vehicle 12.

The motor 36, the generator 16, the power transfer unit 18, and thepower transfer unit 32 may generally be referred to a transaxle 42, ortransmission, of the electric vehicle 12. Thus, when a driver selects aparticular shift position, the transaxle 42 is appropriately controlledto provide the corresponding gear for advancing the electric vehicle 12by providing traction to the vehicle drive wheels 30.

The powertrain 10 may additionally include a control system 44 formonitoring and/or controlling various aspects of the electric vehicle12. For example, the control system 44 may communicate with the electricdrive system 25, the power transfer units 18, 32 or other components tomonitor and/or control the electric vehicle 12. The control system 44includes electronics and/or software to perform the necessary controlfunctions for operating the electric vehicle 12. In one embodiment, thecontrol system 44 is a combination vehicle system controller andpowertrain control module (VSC/PCM). Although it is shown as a singlehardware device, the control system 44 may include multiple controllersin the form of multiple hardware devices, or multiple softwarecontrollers within one or more hardware devices.

A controller area network (CAN) 52 allows the control system 44 tocommunicate with the transaxle 42. For example, the control system 44may receive signals from the transaxle 42 to indicate whether atransition between shift positions is occurring. The control system 44may also communicate with a battery control module of the battery 50, orother control devices.

Additionally, the electric drive system 25 may include one or morecontrollers 54, such as an inverter system controller (ISC). Thecontroller 54 is configured to control specific components within thetransaxle 42, such as the generator 16 and/or the motor 36, such as forsupporting bidirectional power flow. In one embodiment, the controller54 is an inverter system controller combined with a variable voltageconverter (ISC/VVC).

FIG. 2 illustrates part of the electric drive system 25 of the electricvehicle 12 of FIG. 1. The controller 54 includes a plurality ofswitching units 60, such as integrated gate bipolar transistors, thatselectively block current to the generator 16 and/or the motor 36. Theswitching units 60 support bidirectional power flow to and from thegenerator 16 and the motor 36.

Referring to FIG. 3, electric machines such as the generator 16 and themotor 36 of FIG. 1 may include a rotor 56 (or shaft) that rotates togenerate torque. The rotor 56 can be mathematically represented relativeto a 3-phase stationary frame a, b, and c. The 3-phase stationary framea, b and c may be represented in 2-D via a stationary d, q frame and arotating d, q frame. For example, the stationary d, q frame includes adirect axis d_(s) and a quadrature axis q_(s), and the rotating d, qframe includes a direct axis d_(r) and a quadrature axis q_(r). Therotating d, q frame is aligned with movement of the rotor 56. Therefore,θ_(r) represents an angular positioning of the rotor 56. It may becomenecessary during certain conditions of the electric vehicle 12 tocalculate the angular position θ_(r) of the rotor 56 in order to meetelectric control requirements of the electric machine and/or to avoidinaccurate torque production.

FIG. 4 illustrates a rotor position offset detection system 58 that canbe incorporated into an electric vehicle, such as the electric vehicle12 shown in FIG. 1. The rotor position offset detection system 58determines rotor position offset of an electric machine 16, 36 (motorand/or generator). In one embodiment, the rotor position offsetdetection system 58 includes a sensor 62, a control unit 64, a variablevoltage converter 66, and an inverter 68. The control unit 64, thevariable voltage converter 66 and the inverter 68 may be part of thecontroller 54 or could be separate from the controller 54.

The sensor 62 may be a resolver, encoder, speed sensor, or anotherposition sensor that is associated with the electric machine 16, 36. Thesensor 62 monitors an angular position of the rotor 56 (or shaft) of theelectric machine 16, 36. The sensor 56 may be mounted to or separatefrom the rotor 56. The sensor 56 communicates information to the controlunit 64, such as rotor position information concerning the rotor 56.

The rotor position offset detection system 58 may use algorithmsprogrammed into the control unit 64 to apply special voltage commandsand use special processing of the feedback signals to determine anyrotor position offset between the readings from the sensor 62 and anactual positioning of the rotor 56. For example, the control unit 64 maycontrol 3-phase current in the electric machine 16, 36 by commanding3-phase voltages Vabc to the inverter 68 and measuring the 3-phasecurrent Iabc and rotor position θ_(r) as feedback form the inverter 68and the electric machine 16, 36, respectively. This information may becompared to the information from the sensor 62 to determine whether arotor position offset exists. Rotor position offset may result ininaccurate torque output. The variable voltage converter 66 may be usedto convert a control signal to an appropriate voltage level forcontrolling the inverter 68, among other components.

The rotor position offset detection system 58 may additionally include avoltage sensor 69. The voltage sensor 69 is configured to measure avoltage across the windings b, c that extend between the inverter 68 andthe electric machine 16, 36.

A variety of methods or techniques can be used to calculate rotorposition offset in an electric machine, such as by using the rotorposition offset detection system 58 of FIG. 4. FIG. 5, with continuedreference to FIGS. 1-4, schematically illustrates one exemplary method100 of determining rotor position offset of an electric machine, such asthe generator 16, the motor 36 or some other electric machine of theelectric vehicle 12. The method 100 may be performed “in-vehicle,” orwithout removing the transaxle 42 from the electric vehicle 12, and doesnot require spinning the rotor 56 (i.e., without the need to generateany torque or motion within the electric machine 16, 36). The method 100may be referred to as a self-sensing signal injection method.

The self-sensing signal injection method 100 begins at step 102 bydetecting a position of the direct axis d of the rotor 56 of an electricmachine. For example, the position of the direct axis d of the rotor 56may be determined by applying a first voltage of a first magnitude tothe electric machine and then analyzing a current response from theelectric machine to determine the position or alignment of the directaxis d. The first voltage may be a rotating voltage having a relativelyhigh frequency, such as between 100 Hz and 500 Hz. In one embodiment,the current response from the electric machine is analyzed by processingthe negative sequence current response from the electric machine inorder to determine the alignment of the d axis (i.e., permanent magnetaxis).

Once position or alignment is known, the polarity of the direct axis dof the rotor 56 must be determined At step 104, a second voltage of asecond magnitude is applied to the quadrature axis q of the rotor 56 toproduce a current response along the direct axis d. In one embodiment,the second voltage is a different magnitude than the first voltage. Thesecond voltage may be applied to the quadrature axis q using asinusoidal pulsing voltage, which could include either a standard orsquare wave. The current response along the direct axis d is averaged atstep 106 to determine the polarity of the direct axis d.

At step 108, the polarity value of the direct axis d of the rotor 56 isanalyzed. For example, if the polarity calculated at step 106 ispositive, the position of the direct axis d is considered correct.Alternatively, if the polarity is negative, the position calculation isadjusted by 180° to obtain the correct position of the direct axis d ofthe rotor 56.

At step 110, the position information of the direct axis d of the rotor56 collected at step 108 is compared to information from the sensor (orresolver) that monitors a position of the rotor 56 to calculate whethera rotor position offset is out of range. Finally, at step 112, acorrective action is taken if it is determined that a rotor positionoffset is out of range. Exemplary corrective actions include correctingthe offset (i.e., aligning the rotor 56 back to the zero position) andcontinuing operation of the electric machine of the electric vehicle 12,setting a diagnostic troubleshooting code, and/or entering a limitedoperating mode of the electric vehicle 12.

In one embodiment, the method 100 is performed in response to apre-defined prompt. For example, the method 100 can be performed atleast at every key-on condition of the electric vehicle 12. In anotherembodiment, the method 100 can be performed in response to detecting arotor speed that is within a specified range of speeds. In anotherembodiment, the method 100 is performed in response to a pre-programmedcurrent command range. In yet another embodiment, the method 100 can beperformed at specified intervals, such as a specific amount of time ordistance the electric vehicle 12 has been operated. The pre-definedprompt may additionally be related to an electric machine reset orservicing condition.

FIG. 6 schematically illustrates another embodiment of a method 200 fordetermining rotor position offset of an electric machine. The method 200may be performed “in-vehicle,” i.e., without removing the transaxle 42from the electric vehicle 12. The method 200 may include spinning therotor 56; however, the method 200 may be performed without the need togenerate any torque or motion within the electric machine 16, 36. Themethod 200 may be referred to as the voltage command tracking method.

The voltage command tracking method 200 may begin at step 202 byoptionally spinning the rotor 56 of the electric machine at a speed thatis between a minimum speed and a maximum speed of the electric machine.The method 200 may be executed during the normal course of vehicleoperation and whenever certain conditions are met (i.e., speed withincertain range, current command to zero, etc.). The rotor 56 may be spunin a variety of ways. In one embodiment, the rotor 56 may be spunwithout moving the vehicle drive wheels 30, such as by using the engine14 to drive the generator 16 or the motor 36. In another embodiment, therotor 56 is spun by moving the vehicle drive wheels 30 (the electricvehicle 12 may move or be hoisted), such as by driving an electricmachine with the engine 14, driving the motor 36 with both the engine 14and the generator 16, or using a service tool to spin the vehicle drivewheels 30. Other methods may also be utilized to spin the rotor 56 of anelectric machine.

Next, at step 204, the current of the electric machine is activelyregulated to zero. Regulating the current in this way cancels backelectromotive force (EMF) associated with the electric machine. Thevoltage command angle that is used to achieve zero current can then befiltered or averaged at step 206. The voltage command angle can below-pas filtered or averaged over a calibratable time window. At step208, the voltage command angle may be adjusted by a calibratable valueto obtain the rotor position. For example, 90° may be subtracted/addedfrom the voltage command angle to obtain the rotor position. Forexample, 90° may be added to the voltage command angle if the speed isdetermined to be negative, or could be subtracted from the voltagecommand signal if the speed is positive.

At step 210, the rotor position information is compared to informationfrom the sensor (or resolver) that monitors the position of the rotor 56to calculate whether a rotor position offset is out of range. Finally,at step 212, a corrective action is taken if it is determined that arotor position offset is out of range.

FIG. 7 schematically illustrates another exemplary method 300 fordetermining rotor position offset of an electric machine. The method 300may be performed “in-vehicle,” i.e., without removing the transaxle 42from the electric vehicle 12, and optionally requires spinning the rotor56. However, like the methods 100, 200, the method 300 can be performedwithout the need to generate any torque or motion within the electricmachine 16, 36. The method 300 may be referred to as the backelectromotive force (EMF)-to-position signal comparison method.

The method 300 may begin at step 302 by optionally spinning the rotor 56of the electric machine. The rotor 56 may be spun at a speed that isbetween a minimum speed and a maximum speed of the electric machine.Similar to the method 200, the rotor 56 may be spun with or withoutmoving the vehicle drive wheels 30.

The switching units 60 of the controller 54 may be disabled at step 304.In one embodiment, the switching units 60 are disabled by not applyingvoltage signals to their gate drivers.

Next, at step 306, the line-line voltage across the B & C (or V & W)terminals of the three phase electric machine is measured. In oneembodiment, a tool, such as the voltage sensor 69 (see FIG. 4), is usedto perform the measuring step. A positive-slope zero-crossing of theline-line voltage is estimated using the tool at step 308. The positionsensor reading at the positive slope zero-crossing is representative ofthe rotor position error (i.e., rotor position offset). Finally, at step310, a corrective action is taken if it is determined that a rotorposition offset is out of range.

FIG. 8 schematically illustrates yet another method 400 for determiningrotor position offset of an electric machine. The method 400 may beperformed “in-vehicle” and requires minimal movement of the rotor 56.The method 400 may be referred to as the decaying sinusoidal torquemethod. In the decaying sinusoidal torque method 400, a decayingsinusoidal torque is applied to the electric machine to move it into acertain position.

At step 402, the transaxle 42 of the electric vehicle 12 is optionallydecoupled from a road or other traction surface, such as by hoisting theelectric vehicle 12. The method 400 then continues to step 404 byapplying a first current of a first magnitude to the direct axis d ofthe electric machine. The first current may be a constant current, inone embodiment. Next, at step 406, a second current of a secondmagnitude and frequency is applied to the quadrature axis q of theelectric machine. The second current of the quadrature axis q is taperedto a configurable amplitude at a calibratable taper rate, such as byusing a linear ramp or an exponential decay, at step 408. This causesthe rotor 56 to oscillate around and decay toward a zero position of theelectric machine. At step 410, the position response of the rotor 56 isfiltered/averaged to obtain a zero position reading, thereby enablingcalculation of the rotor position offset.

Although the different non-limiting embodiments are illustrated ashaving specific components or steps, the embodiments of this disclosureare not limited to those particular combinations. It is possible to usesome of the components or features from any of the non-limitingembodiments in combination with features or components from any of theother non-limiting embodiments.

It should be understood that like reference numerals identifycorresponding or similar elements throughout the several drawings. Itshould be understood that although a particular component arrangement isdisclosed and illustrated in these exemplary embodiments, otherarrangements could also benefit from the teachings of this disclosure.

The foregoing description shall be interpreted as illustrative and notin any limiting sense. A worker of ordinary skill in the art wouldunderstand that certain modifications could come within the scope ofthis disclosure. For these reasons, the following claims should bestudied to determine the true scope and content of this disclosure.

What is claimed is:
 1. A method, comprising: detecting a position,including a polarity, of a rotor to detect incorrect rotor positionoffset of an electric machine without generating torque or motion withinthe electric machine.
 2. The method as recited in claim 1, wherein thestep of detecting includes: applying a voltage; and analyzing a currentresponse from the electric machine to determine the position, includingthe polarity, of the rotor.
 3. The method as recited in claim 2, whereinthe step of detecting includes: applying a first voltage of a firstmagnitude and a first frequency to the electric machine to produce afirst current response; processing a first current response from theelectric machine to determine an alignment of a direct axis of therotor; applying a second voltage of a second magnitude and a secondfrequency to the electric machine; and analyzing a second currentresponse from the electric machine to determine the polarity of thedirect axis of the rotor.
 4. The method as recited in claim 3, whereinthe first magnitude is a different magnitude from the second magnitude.5. The method as recited in claim 3, wherein the first frequency is adifferent frequency from the second frequency.
 6. The method as recitedin claim 3, wherein the step of processing includes processing anegative sequence current response from the electric machine.
 7. Themethod as recited in claim 3, wherein the first voltage is a sinusoidalrotating voltage and the second voltage is a sinusoidal pulsing voltagealong the direct axis.
 8. The method as recited in claim 3, comprisingone of the steps of: determining that the polarity of the direct axis iscorrect if the average value of the first current response is a positivevalue; or determining that the polarity of the direct axis is incorrectif the average value of the first current response is a negative value.9. The method as recited in claim 8, comprising the step of adjustingthe position by 180° if the average value of the first current responseis the negative value.
 10. The method as recited in claim 3, comprisingthe step of comparing the position and the polarity of the direct axisof the rotor to information from a sensor configured to monitor theelectric machine to determine the incorrect rotor position offset. 11.The method as recited in claim 10, comprising the step of taking acorrective action if the rotor position offset is out of range.
 12. Themethod as recited in claim 1, wherein the step of detecting is performedin response to a predefined prompt.
 13. The method as recited in claim12, wherein the predefined prompt is a key-on condition of an electricvehicle.
 14. The method as recited in claim 1, wherein the step ofdetecting includes using a voltage command tracking method.
 15. Themethod as recited in claim 1, wherein the step of detecting includescomparing back electromotive force (EMF) to position signal.
 16. Amethod, comprising: detecting an incorrect rotor position offset of anelectric machine using a decaying sinusoidal torque.
 17. The method asrecited in claim 16, wherein using the decaying sinusoidal torqueincludes: applying a first current of a first magnitude to a direct axisof a rotor of the electric machine; applying a second current of asecond magnitude to a quadrature axis of the rotor; tapering the secondcurrent to a configurable amplitude at a calibratable taper rate; andfiltering a position response of the rotor to identify a zero position.18. The method as recited in claim 17, wherein the step of taperingincludes using one of a linear ramp and an exponential decay.
 19. Arotor position offset detection system, comprising: an electric machinehaving a rotor; a sensor that monitors a position of said rotor; acontrol unit in communication with said sensor; and an inverter incommunication with said control unit, said control unit configured tocompare information from said sensor with feedback from said inverter todetect an incorrect rotor position offset of said rotor.
 20. The systemas recited in claim 19, wherein said control unit is configured tocommand a 3-phase voltage to said inverter.